The instant invention generally relates to proximity sensors, and more particularly to proximity sensors that are responsive to an electric-field-influencing media.
A variety of systems have been developed to suppress the passenger air bag in dangerous situations. Systems have included sensors used alone or in combinations. Sensor technologies used include:
a) Infra-red sensors
b) Passive infra-red sensors (heat detectors)
c) Ultrasonic sensors
d) Capacitive sensors (usually using a different type of capacitive sensor)
e) Weight sensors (including various sensor technologies and measurement methods)
f) Child seat xe2x80x9ctagxe2x80x9d sensors
Usually two or more of these sensors are used together in an attempt to identify child seats, small occupants, empty seats, large occupants and out-of-position occupants. The more sensors that are used, the better the chance for a high performance system. The costs of systems that use many sensors however, can become prohibitively high because of the large number of components and the increased assembly complexity of the vehicle.
The prior art also teaches the use of capacitive sensing to detect the presence, proximity, or position of an occupant. U.S. Pat. No. 3,740,567 teaches the use of electrodes incorporated into the base and back of the seat respectively, together with a capacitance responsive circuit, for purposes of discriminating between human occupants and animals or packages resting on an automobile seat. U.S. Pat. No. 3,898,472 teaches an occupant detection apparatus which includes a metallic electrode which is disposed to cooperate with the body of an automobile to form an occupant sensing capacitor, together with related circuitry which senses variations in the associated capacitance responsive to the presence of an occupant. U.S. Pat. No. 4,300,116 teaches the use of a capacitive sensor to detect people proximate the exterior of a vehicle. U.S. Pat. No. 4,796,013 teaches a capacitive occupancy detector wherein the capacitance is sensed between the base of the seat and the roof of the vehicle. U.S. Pat. No. 4,831,279 teaches a capacity responsive control circuit for detecting transient capacitive changes related to the presence of a person. U.S. Pat. Nos. 4,980,519 and 5,214,388 teach the use of an array of capacitive sensors for detecting the proximity of an object. U.S. Pat. No. 5,247,261 teaches the use of an electric field responsive sensor to measure the position of a point with respect to at least one axis. U.S. Pat. No. 5,411,289 teaches the use of a capacitive sensor incorporated into the back rest of the seat to detect occupant presence. U.S. Pat. No. 5,525,843 teaches the use of electrodes incorporated into the base and back of the seat for purpose of detecting the presence of an occupant, whereby the electrodes are substantially insulated from the vehicle chassis when the detection circuit is active. U.S. Pat. Nos. 5,602,734 and 5,802,479 teach an array of electrodes mounted above the occupant for purposes of sensing occupant position based upon the influence of the occupant on the capacitance amongst the electrodes. U.S. Pat. No. 5,166,679 teaches a capacitive proximity sensor with a reflector driven at the same voltage as to sensing element to modify the sensing characteristic of the sensor. U.S. Pat. No. 5,770,997 teaches a capacitive vehicle occupant position sensing system wherein the sensor generates a reflected electric field for generating an output signal indicative of the presence of an object. U.S. Pat. Nos. 3,943,376, 3,898,472, 5,722,686, and 5,724,024 also teach capacitive-based systems for sensing occupants in motor vehicles.
In addition to methods taught by the above referenced U.S. Patents, the prior art also teaches various means of measuring capacitance, as for example given in the Standard Handbook for Electrical Engineers 12th edition, D. G. Fink and H. W. Beaty editors, McGraw Hill, 1987, pp. 3-57 through 3-65 or in Reference Data for Engineers: Radio, Electronics, Computer, and Communications 7th edition, E. C. Jordon editor in chief, Howard W. Sams, 1985, pp. 12-3 through 12-12, both included herein by reference.
The technical paper xe2x80x9cField mice: Extracting hand geometry from electric field measurementsxe2x80x9d by J. R. Smith, published in IBM Systems Journal, Vol. 35, Nos. 3 and 4, 1996, pp. 587-608, incorporated herein by reference, describes the concept of Electric Field Sensing as used for making non-contact three-dimensional position measurements, and more particularly for sensing the position of a human hand for purposes of providing three dimensional positional inputs to a computer. What has commonly been referred to as capacitive sensing actually comprises the distinct mechanisms of what the author refers to as xe2x80x9cloading modexe2x80x9d, xe2x80x9cshunt modexe2x80x9d, and xe2x80x9ctransmit modexe2x80x9d which correspond to various possible electric current pathways. In the shunt mode, a voltage oscillating at low frequency is applied to a transmit electrode, and the displacement current induced at a receive electrode is measured with a current amplifier, whereby the displacement current may be modified by the body being sensed. In the xe2x80x9cloading modexe2x80x9d, the object to be sensed modifies the capacitance of a transmit electrode relative to ground. In the transmit mode, the transmit electrode is put in contact with the user""s body, which then becomes a transmitter relative to a receiver, either by direct electrical connection or via capacitive coupling.
There are many technologiesxe2x80x94including ultrasonic, active infrared, and passive infraredxe2x80x94that have been used for sensing the position of an occupant in a motor vehicle. Some of the problems associated with ultrasonic sensors include: poor reflections off some clothing types, relatively slow pulse propagation times, susceptibility to acoustic noise, and blockage by a newspaper. Some of the problems associated with active infrared sensors include: poor reflections off some clothing types, signal saturation as a result of extreme sunlight conditions, and blockage by a newspaper. Some of the problems associated with passive infrared sensors include: poor signal as a result of occupant""s clothing, poor contrast because of ambient temperature, and difficulty in making direct distance measurements.
In an elementary capacitive sensor a metal electrode is connected to a capacitance measuring circuit. Whereas many variations of capacitive sensors are taught in the related art for the occupant sensing application, one problem with known capacitive sensors is a relatively low signal to noise ratio that can be caused by uncertainty in measurement offsets, particularly when sensing very low levels of capacitance, for which the noise resulting from signal drift can overwhelm the signal in static situations where the target is at a distance that would cause only a small change in the capacitance. This can limit the useful range of position measurements that can be reliably sensed by the sensor. Drifts in the offset may result from drift in the measurement electronics or in the connection to the sensing electrode. Furthermore, known single electrode capacitive sensors measure a single value representing a summation of the total capacitance to ground in all directions, including to the rear of the sensor, from which it is difficult or impossible to obtain an accurate idea of an object""s location relative to sensor.
The instant invention overcomes the above-noted problems by providingxe2x80x94in a first aspectxe2x80x94a proximity sensor for sensing an electric-field-influencing media within a region of space, comprising a first electrode, at least one second electrode, a first oscillatory signal operatively connected to the first electrode, at least one second oscillatory signal operatively connected to the at least one second electrode, and a circuit operatively connected to the first electrode for sensing a third signal from the first electrode and for generating a measure of the proximity of the electric-field-influencing media to the first electrode. The first electrode functions as a sense electrode, and the at least one sense electrode functions as at least one control electrode.
The first and at least one second electrodes are conductive and are preferably disposed on a common, first surface, wherein the at least one second electrode is disposed outside a periphery of the first electrode. The at least one second electrode is located proximate to the first electrode so that an electric field between the first electrode and the at least one second electrode occupies the region of space to be sensed. The electric field is generated by the first and at least one second electrodes responsive to the first and the at least one second oscillatory signals, the latter of which provides for varying the geometry of the electric field.
The at least one second oscillatory signal is coherent with the first oscillatory signal, and comprises a plurality of second signal components, each component comprising a specific signal level or signal phase at a distinct time.
Preferably, the at least one second electrode comprises a plurality of second electrodes, each isolated from one another, all surrounding the first electrode, and the corresponding at least one second oscillatory signal comprises a plurality of second oscillatory signals that are operatively connected to respective second electrodes of the plurality of second electrodes. In one embodiment, the second signal components are either in-phase or out-of-phase relative with the first oscillatory signal. Moreover, the respective series of second signal components to the respective second electrodes are independent of one another with respect to either signal level or signal phase.
The third signal comprises a plurality of third signal components, each of which is respectively responsive to the first oscillatory signal, the respective second signal component, and to a proximity to the first electrode of the electric-field-influencing media For the first and at least one second oscillatory signals comprising voltage signals, the third signal comprises a current signal. Alternately, for the first and at least one second oscillatory signals comprising current signals, the third signal comprises a voltage signal. Generally, the third signal provides a measure of the capacitance of the first electrode. The circuit generates a plurality of measures from the plurality of third signal components, and generates a likelihood of a proximity scenario responsive to the plurality of measures, wherein the proximity scenario is indicative of a proximity of the electric-field-influencing media to the first electrode. Preferably, the circuit generates a respective plurality of the likelihoods for a respective plurality of proximity scenarios, and selects a most likely proximity scenario from the plurality of proximity scenarios.
In accordance with the first aspect, the instant invention provides a method of sensing the proximity of an electric-field-influencing media within a region of space comprising applying a first oscillatory signal to a first electrode, applying at least one second oscillatory signal to a respective at least one second electrode, and sensing a third signal from the first electrode. The first electrode functions as a sense electrode and the at least one second electrode functions as at least one control electrode. The at least one second oscillatory signal comprises a plurality of second signal components each comprising a distinct signal level or signal phase relative to a phase at a distinct time. Preferably a plurality of second oscillatory signals are applied to a respective plurality of second electrodes. The third signal comprises a plurality of third signal components, each respectively responsive to a different respective second signal component. The method further comprises generating a plurality of measures from the plurality of third signal components, establishing a plurality of proximity scenarios, generating a plurality of likelihoods from the plurality of measures, and selecting a most likely proximity scenario from the plurality of proximity scenarios. Each proximity scenario comprises a different proximity, or location, of the electric-field-influencing media to the first electrode, and each respective likelihood is the likelihood of the respective proximity scenario.
The control electrodes provide for modifying the geometry of the electric field of the proximity sensor, so as to enable objects to be to be detected proximate in all directions relative to the sensor, including above or below, or left or right depending upon the control electrode configuration, wherein the electric field created by the sense and control electrodes is modified by the control signals so as to probe the space proximate to the proximity sensor. The response to a particular electric field configuration is measured from the current of the voltage signal applied to the sense electrode. Different electric field configurations provide for different associated sensitivity patterns of the capacitance of the sense electrode to a surrounding ground, responsive to objects in a region of space proximate to the proximity sensor, wherein the objects comprise electric-field-influencing media. By measuring the capacitance of the sense electrode for the control electrodes at various potentials relative to the sensor electrode, a plurality of independent capacitance values are obtained, from which, in combination, the spatial location of an object proximate to the sensor is determined relative to a set of a priori proximity scenarios using a probabilistic algorithm employing Bayes rule. The type and location of an object sensed by the proximity sensor is given by the object type and location associated with the most likely proximity scenario determined from the combination of capacitance measurements.
Larger numbers of independent capacitance measurements and a priori proximity scenarios provide for a more detailed measurement of the type and location of an unknown target or targets proximate to the proximity sensor.
Changes of capacitance resulting from target movement can also be used to increase confidence in the measured target location, because the sensitivity of the capacitance of the sense electrode to changes in object location is generally different for different electric field configurations. Moreover, changes in capacitance are relatively easy to measure, because they are not affected by capacitance offsets in the system. However, target movement, which cannot be controlled, is not generally suitable for the initial proximity measurements.
Depending on the application, the sensor response may be optimized by changing the sense electrode size, the number and sizes of the control electrodes, and/or the gap width.
In accordance with a second aspect of the instant invention, a proximity sensor for sensing an electric-field-influencing media within a region of space comprises a first electrode having first and second sides, a first oscillatory signal operatively connected to the first electrode; and a third electrode proximate to and insulated from the first side of the first electrode, located between the first side of the first electrode and the region to be sensed. The first and third electrodes are each conductive, are disposed on respective first and second surfaces, and preferably have substantially equal boundaries.
In a shielding mode, the third electrode is operatively connected to the first oscillatory signal. In a sensing mode, a third signal from the first electrode is responsive to the first oscillatory signal and to a proximity to the first side of the first electrode of the electric-field-influencing media. For the first signal comprising a voltage signal, the third signal comprises a current signal. Alternately, for the first signal comprising a current signal, the third signal comprises a voltage signal. Generally, the third signal provides a measure of the capacitance of the first electrode. In the sensing mode, the third electrode is either operatively connected to the first electrode, or is electrically floating.
The second aspect further provides for a fourth electrode proximate to and insulated from the second side of the first electrode, having a boundary that extends up to and preferably beyond the boundary of the first electrode. The fourth electrode is conductive, and is disposed on a third surface, wherein the second and third surfaces are preferably substantially parallel to the first surface. The fourth electrode is operatively coupled to the first oscillatory signal so as to shield the first electrode from the influence of an electric-field-influencing media proximate to the second side of the first electrode.
In accordance with the second aspect, the instant invention provides a method of sensing the proximity of an electric-field-influencing media within a region of space comprising applying a first oscillatory signal to a first electrode having first and second sides; in a shielding mode, applying the first oscillatory signal to a third electrode, wherein the third electrode is located between the first side of the first electrode and the region of space; and in a sensing mode, sensing a signal from the first electrode responsive to the proximity of an electric-field-influencing media to the first electrode, wherein the sensing and the shielding modes are mutually exclusive. The first electrode functions as a sense electrode, and the third electrode functions as a front driven shield. In the sensing mode, the third electrode is either operatively connected to the first electrode or is isolated from the first electrode. The method further comprises applying the first oscillatory signal to a fourth electrode located proximate to the second side of the first electrode, wherein the fourth electrode functions as a rear driven shield.
The front and rear driven shields effectively cancel the capacitance of the sense electrode to ground, on the side of the sense electrode where the driven shield is operative. This cancellation results because both the sense electrode and the driven shield have the same potential so as to prevent any displacement current therebetween. The rear driven shield cancels the capacitance of the sense electrode with objects or grounds behind the sense electrode. The front driven shield cancels the capacitance of the sense electrode with objects or grounds in the target region in front of the sensor, thereby enabling the sensor to ignore targets and thereby enable the associated circuit to characterize the associated parasitic capacitance during a calibration process using a known capacitance of an associated calibration capacitor. The calibration capacitor may be a discrete capacitor in the circuit, or it could be a capacitance from the sensor, e.g. a portion of the front driven shield that could be grounded. This calibration process extends the useful range of the proximity sensor. Moreover, when used in conjunction with the front and rear driven shields, this calibration process can be conducted at various times so as to characterize and compensate for drift of parasitic capacitance and circuit gain, or to identify system failures. The front driven shield is either connected to the sense electrode or is electrically floating during the measurement process, wherein the electric-field-influencing media being sensed respectively influences the sense electrode by either conduction or induction.
Accordingly, one object of the instant invention is to provide a proximity sensor with an expanded sensing range.
A further object of the instant invention is to provide a proximity sensor with reduced sensitivity to parasitic capacitance and to associated drifts in parasitic capacitance and circuit gain over time.
A yet further object of the instant invention is to provide a proximity sensor with an improved signal to noise ratio.
A yet further object of the instant invention is to provide a proximity sensor that can determine the presence and location of electric-field-influencing media proximate to the sensor.
A yet further object of the instant invention is to provide a proximity sensor that is insensitive to electric-field-influencing media located behind the sensor, outside the sensing region.
The specific features of the instant invention provide a number of associated advantages. One advantage of the instant invention with respect to the prior art is that the front and rear driven shields enable the proximity sensor to be calibrated so as to compensate for parasitic capacitance and gain drift, thereby improving the signal to noise ratio and range of associated proximity measurements.
Another advantage of the instant invention is that the feature of making all capacitance measurements from a single sense electrode simplifies the associated measurement circuit and improves accuracy by eliminating the need for a switching network between the measurement circuit and the various electrodes.
Yet another advantage of the instant invention is that by applying a plurality of control signals to at least one control electrode and thereby generate a plurality of different electric field configurations, each providing a distinct response from the sense electrode to an object within the electric field, the instant invention provides a plurality of capacitance measurements by which the object can be located relative to the proximity sensor.
Yet another advantage of the instant invention is that by measuring the proximity of objects using a sensor responsive to the electric-field-influencing properties of an object, the instant invention is relatively insensitive to objects such as newspapers or other obstructions that have relatively little influence on an electric field, but which would otherwise affect other proximity sensors such as ultra-sonic or infra-red ranging sensors.
When applied as an occupant sensor in a safety restraint system, for example when mounted proximate to an air bag inflator so as to sense the region of space proximate thereto, the spatial and distance measurements from the proximity sensor of the instant invention provide for a determination of whether an occupant is in an xe2x80x9cat-risk zonexe2x80x9d proximate to the air bag inflatorxe2x80x94and thereby at risk of injury from a deployment thereofxe2x80x94so that the air bag inflator may be disabled if an occupant is within the xe2x80x9cat-risk zonexe2x80x9d.
These and other objects, features, and advantages of the instant invention will be more fully understood after reading the following detailed description of the preferred embodiment with reference to the accompanying drawings and viewed in accordance with the appended claims.